He might make circular plug, trailer or fancy driver seat (10)
I believe the answer is:
advertiser
'he might make circular plug trailer' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'fancy driver seat' is the wordplay.
'fancy' indicates anagramming the letters.
'driver'+'seat'='driverseat'
'driverseat' with letters rearranged gives 'ADVERTISER'.
'or' is the link.
Can you help me to learn more?
(Other definitions for advertiser that I've seen before include "Publicist" , "One publicising a product (S, not Z)" , "Commercial producer" , "He pushes" , "Part of a newspaper's title" .)